import java.util.Arrays;

public class MyStack2<E> {
    //模拟实现一个泛型的栈
    public Object[] elem;
    public int usedSize;
    public MyStack2() {
        this.elem=new Object[10];
    }
    public void push(E val) {
        if (isFull()) {
            //满了则对它进行扩容
            elem = Arrays.copyOf(elem, 2 * elem.length);
        }
        elem[usedSize] = val;
        usedSize++;
    }

    public boolean isFull() {
        return usedSize == elem.length;
    }

    public E pop() {
        if (empty()) {
            return null;
        }
        E oldVal = (E) elem[usedSize - 1];
        usedSize--;
        return oldVal;
    }
    public E peek() {
        if (empty()) {
            return null;
        }
        return (E) elem[usedSize - 1];
    }

    public boolean empty() {
        return usedSize == 0;
    }
}
